The proposed project will initiate the first longitudinal twin study of reading disability and conduct novel analyses of the genetic and environmental etiologies of stability and change in reading difficulties. To accomplish these objectives, an extensive psychometric test battery, as well as behavioral questionnaires and interviews, will be administered to a sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair is reading disabled, to their siblings, and to a comparison sample of 50 pairs of identical twins and 50 pairs of fraternal twins with no history of reading deficits, all of whom previously participated in the Colorado Learning Disabilities Research Center CLDRC). Resulting data will be analyzed in conjunction with data previously collected in the CLDRC to provide the first longitudinal assessment of the genetic and environmental etiologies of reading deficits and subtypes of reading disability at two ages, as well as their longitudinal stability and comorbidity with ADHD. These data will also be used to test novel hypotheses about the longitudinal covariation of reading difficulties with measures of other psychopathology, reading, language and perceptual processes and specific cognitive abilities. Univariate and bivariate QTL analyses of reading deficits and ADHD, reading component processes and other psychopathology will also be conducted.